Leadless pacing device implantation

ABSTRACT

In some examples, a system includes an implantable medical device configured for implantation in a chamber of the heart, an extension attached to the implantable medical device, the extension comprising a housing comprising at least one electrode, the housing defining a hole, and a tether comprising a first tether portion and a second tether portion and configured to be threaded through the hole. When the tether is threaded through the hole, the first tether portion and the second tether portion are on opposite sides of the hole. The tether may be used to implant the extension in a different chamber of the heart of the patient than the implantable medical device.

TECHNICAL FIELD

The disclosure relates to cardiac pacing, and more particularly, tocardiac pacing using a pacing device implanted within the heart.

BACKGROUND

An implantable pacemaker may deliver pacing pulses to a patient's heartand monitor conditions of the patient's heart. In some examples, theimplantable pacemaker comprises a pulse generator and one or moreelectrical leads. The pulse generator may, for example, be implanted ina small pocket in the patient's chest. The electrical leads may becoupled to the pulse generator, which may contain circuitry thatgenerates pacing pulses and/or senses cardiac electrical activity. Theelectrical leads may extend from the pulse generator to a target site(e.g., an atrium and/or a ventricle) such that electrodes at the distalends of the electrical leads are positioned at a target site. The pulsegenerator may provide electrical stimulation to the target site and/ormonitor cardiac electrical activity at the target site via theelectrodes.

A leadless pacing device has also been proposed for sensing electricalactivity and/or delivering therapeutic electrical signals to the heart.The leadless pacing device may include one or more electrodes on itsouter housing to deliver therapeutic electrical signals and/or senseintrinsic depolarizations of the heart. The leadless pacing device maybe positioned within or outside of the heart and, in some examples, maybe anchored to a wall of the heart via a fixation mechanism.

SUMMARY

The disclosure describes devices, systems, and methods for implanting acardiac pacing system that includes an implantable medical device(hereinafter, “IMD”) configured for implantation within a chamber of aheart of a patient, and an extension extending from a housing of theIMD, which is also configured to be implanted within the heart. Theextension includes one or more electrodes with which the IMD may senseelectrical cardiac activity and/or provide electrical stimulation. Theextension is electrically coupled to a sensing module and/or astimulation module of the IMD. In some examples, the IMD is configuredto be implanted in a first chamber of a heart of a patient (e.g., theright ventricle), and a portion of the extension may be configured toposition at least one electrode within or proximate to another chamberof the heart (e.g., the left ventricle), thereby allowing the IMD todeliver biventricular pacing in a patient or to sense electricalactivity in at least two chambers of the heart.

In some example, the extension includes a housing that includes one ormore electrodes configured to be positioned at a target site within theheart that is within or proximate to a chamber of the heart other thanthe one in which the IMD is implanted. The housing may define anopening, through which a tether may be threaded. The tether, inconjunction with a guidewire and a pusher, may be used to guide thehousing to the target site. In some examples, a proximal end (or aproximal portion) of the guidewire is mechanically connected to a distalend of the tether, such that as the tether is pulled through the holedefined by the housing in a proximal direction, the guidewire isthreaded through the hole. In this way, the housing may be threadedthrough the guidewire and may be guided to the target site via theguidewire.

In some examples, the tether may also be threaded through an eyelet orother opening defined by a pusher. As the tether is pulled through thehole defined by the housing in a proximal direction and the guidewire isthreaded through the hole, the guidewire is also threaded through theeyelet defined by the pusher. The pusher is configured to engage with aproximal side of the housing to apply a force to push the housing in adistal direction, towards the target site.

In one example, the disclosure is directed to a system comprising animplantable medical device configured to be implanted in a chamber of aheart of a patient; an extension attached to the implantable medicaldevice, the extension comprising a housing comprising at least oneelectrode, the housing defining a hole; and a tether comprising a firsttether portion and a second tether portion and configured to be threadedthrough the hole, wherein when the tether is threaded through the hole,the first tether portion and the second tether portion are on oppositesides of the hole.

In another example, the disclosure is directed to a method comprisingintroducing an implantable medical device to a first chamber of a heartof a patient via a femoral vein, wherein the implantable medical deviceis attached to an extension comprising a housing including at least oneelectrode, the housing defining a hole, and wherein a tether is threadedthrough the hole, the tether including a first tether portion and asecond tether portion on an opposite side of the hole from the firsttether portion; directing a guidewire to a target vein, the guidewirecomprising a first guidewire portion and a second guidewire portion, thesecond guidewire portion comprising a thread attached to the secondtether portion; introducing a pusher up the second tether portion to aposition proximate to the housing; removing the tether from the patientby at least pulling the second tether portion to replace the tether withthe guidewire; pushing the housing along the guidewire using the pusher.

The details of one or more examples of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the disclosure will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example cardiac pacingsystem that comprises an implantable medical device (IMD) and anextension.

FIG. 2 is a conceptual illustration of another example cardiac pacingsystem that includes an IMD and an extension.

FIG. 3 is a functional block diagram of an example IMD.

FIGS. 4A-4F illustrate various steps of an example method for implantingthe system depicted in FIG. 1.

FIG. 5 is a flow chart illustrating an example method for implanting acardiac pacing system in a heart of a patient.

FIG. 6 is a conceptual illustration of a distal housing of an exampleextension.

FIG. 7 is a conceptual illustration of a cutaway of an distal housing ofan example extension.

FIG. 8 is a conceptual illustration of an example guidewire and anexample stiffener.

DETAILED DESCRIPTION

This disclosure is directed to a system and method for implanting acardiac pacing system capable of providing biventricular pacing to apatient. The system comprises an IMD which is configured to be implantedin a chamber of the heart, and an extension or lead including one ormore electrodes, also configured for placement within the heart, e.g.,in a different chamber than the IMD. In some examples, the system andmethod are configured to modify a leadless pacing device (LPD) in amanner so that it may provide biventricular pacing.

For example, an extension that includes one or more electrodes may bemechanically connected to a housing of the LPD such that it extends fromthe housing, and the one or more electrodes may be electricallyconnected to a sensing module of the LPD, a stimulation module of theLPD, or both the sensing and stimulation modules. In some examples, theextension includes two electrodes, although any suitable number ofelectrodes may be used in other examples. For example, a proximateelectrode (closer to the LPD) may be positioned approximately 3-10centimeters (cm) from the distal end of the LPD and a distal electrodemay be positioned approximately 20-25 cm from the distal end of the LPD.The electrodes of the extension may be used for sensing electricalcardiac signals, delivering electrical stimulation, or both. In someexamples, the LPD may be programmed to sense atrial signals via theproximate electrode and to provide pacing stimulation pulses via thedistal electrode.

In some examples, at least one electrode (e.g., the distal electrode) ofthe extension may be positioned on a housing that is attached to theextension. In some examples the IMD is implanted in the right ventricle.One or more electrodes on the IMD may provide electrical stimulation tothe right ventricle, as well as sense electrical signals from theventricle. In addition, in some examples in which the IMD is implantedin the right ventricle, the distal electrode of the extension may beimplanted in a contrary vein via the coronary sinus. In some examples,the coronary sinus is accessed from the inferior vena cava.

The housing of the extension may include one or more features that aidthe implantation of the housing in a chamber of a heart of the patient,e.g., aid in the control of the housing position relative to thechamber. For example, the housing may define a hole that includes acentral axis that is parallel to the longitudinal axis of the extension.A tether may be installed through the hole. The tether may be positionsuch that a first portion of the tether is one side of the housing and asecond portion of the tether is on the other side of the housing.

The tether may help control the position of the housing, and, therefore,aid in proper positioning of the extension relative to a targetstimulation site (e.g., a left ventricle). For example, a clinicianimplanting the system in a patient may pull on the tether to help movethe housing of the extension in a first direction, e.g., towards theclinician, and may also use a pusher to move the housing in a seconddirection opposite the first direction. In some examples, the pusher mayinclude an eyelet or another feature at a distal end that is configuredto engage with the housing to apply a force against the housing to movethe housing in the second direction.

In some examples, a guidewire may further aid in the implantation of thedistal electrode of the extension in a patient. In some examples, aguidewire including a first guidewire portion, e.g., which may bedefined by a standard guidewire, and a second guidewire portion. Thesecond guidewire portion may be or may comprise, for example, a suturethread or another material that is more flexible than the firstguidewire portion. A proximal end of the second guidewire portion may beattached to a proximal end of the first tether portion.

In examples in which the second guidewire portion comprises a suturethread, a stiffening tube may be used to stiffen the second guidewireportion while the guidewire is being placed. The stiffening tube can be,for example, formed from a biocompatible plastic or another materialthat is more stiff than the second guidewire portion. A clinician may beguide the guidewire to the target chamber of the heart of the patientusing a catheter that defines a pathway for the guidewire, and, afterthe distal end of the first guidewire portion is placed at the desiredlocation within the patient, the stiffening tube may be removed from thepatient along with the catheter.

A clinician may introduce a pusher into the patient, e.g., by threadinga distal end of the pusher on the second tether portion and then pushingthe pusher along the second tether portion to a position proximate thehousing of the extension. The pusher is relatively stiff and isconfigured to apply a force against the housing of the extension to helpchange the position of the housing within the patient. In some examples,the pusher may be made of wire and may include an eyelet at the end, theeyelet being configured to receive the tether and the guidewire. In someexamples, the eyelet may be closed, such that only the end of the tethermay be threaded through the eyelet. This may be referred to as athreading in a longitudinal direction. In other examples, the eyelet maybe open, such that the tether may be threaded through the eyelet ineither the longitudinal direction or a lateral direction, in which asection of the tether between the proximal and distal ends may beintroduced into the eyelet through the opening.

The pusher may be used to push the electrode through an introducer tothe target position within a heart of the patient, e.g., into the rightatrium or another chamber. At the proximal end of the introducer, theproximal end of the thread may be attached to the first tether portion.A clinician may pull on the second tether portion, e.g., in a directiontowards the clinician to remove the tether from the hole. Because thesecond guidewire portion is mechanically connected to the first tetherportion, once the tether has been removed, the guidewire replaces thetether within the hole in the housing. The clinician may then use pusherto push the housing along the guidewire to the target location withinthe patient.

For fine adjustments to the electrode location once the housingapproaches the end of the guidewire, the clinician may push on thepusher to move the electrode distally (in a direction away from theclinician) or pull on both the guidewire and the pusher to move theelectrode in the proximal direction (in a direction towards theclinician).

A cardiac pacing system consistent with the present disclosure may becontained with the patient's heart once implanted. By locating the IMDand the extension in the heart, the risk of lead fracture along the pathfrom the IMD implant location to the heart is eliminated.

FIG. 1 is a conceptual diagram illustrating an example cardiac pacingsystem 10 that comprises an implantable medical device (IMD) 12 and anextension 14 implanted in patient 22. In some examples, IMD 12 is aleadless pacing device because IMD 12 is not connected to any leads thatextend outside of heart 18. Extension 14 is configured to position afirst electrode (not shown) proximate to or within a chamber of heart 18other than the one in which IMD 12 implanted. In the example shown inFIG. 1, IMD 12 is implanted in right ventricle 16 of heart 18 of patient22. More particularly, IMD 12 is fixed or attached to the inner wall ofthe right ventricle 16 proximate to the apex of the right ventricle inthe example of FIG. 1. In other examples, IMD 12 may be fixed to theinner wall of right ventricle 16 at another location, e.g., on theintraventricular septum or free-wall of the right ventricle, or may befixed to the outside of heart 18, i.e., epicardially, proximate to rightventricle 16. In other examples, IMD may be fixed within, on, or nearthe left-ventricle of heart 18.

In the example shown in FIG. 1, extension 14 is configured to extendaway from IMD 12 and into coronary sinus 26 when IMD 12 is implanted inan apex of right ventricle 16. In some examples, extension 14 may have alength that permits extension 14 to reach a target vein within thecoronary sinus 26 in order for at least one electrode of extension 14 toprovide electrical stimulation to the left ventricle of heart 18. Forexample, extension 14 may have a length of approximately 25 centimeters(cm) (as measured from the proximal end connected to IMD to a distalmost electrode of extension 14). In some examples, extension 14 may havea relatively small cross-sectional dimension (where the cross-section istaken in a direction perpendicular to a longitudinal axis of extension14) and flexible enough to permit the tricuspid valve to sufficientlyclose around extension 14 to prevent backflow into right atrium 24 fromright ventricle 16. For example, extension 14 may be about 4 French(i.e., about 1.33 millimeters (mm)) in diameter. In some examples,extension 14 may include a first portion approximately 4 French indiameter and a second, more distal, portion about 2 French (i.e., about0.66 mm) in diameter.

In the example shown in FIG. 1, IMD 12 is configured to electricalactivity of right atrium 24, right ventricle 16 and/or the leftventricle (via an electrode in the coronary sinus). IMD 12 may also beconfigured to generate and delivering pacing stimulation to both theright and left ventricles. In some examples, the most distal electrode(not shown) of extension 14 is affixed to heart 18 via coronary sinus 26while a second electrode of extension 14 is not affixed to cardiactissue. Extension 14 is configured to continue to extend away from IMD12 towards right atrium 24, even in the presence of blood flow fromright atrium 24 to right ventricle 16.

Also shown in FIG. 1 is medical device programmer 20, which isconfigured to program IMD 12 and retrieve data from IMD 12. Programmer20 may be a handheld computing device, desktop computing device, anetworked computing device, etc. Programmer 20 may include acomputer-readable storage medium having instructions that cause aprocessor of programmer 20 to provide the functions attributed toprogrammer 20 in the present disclosure. IMD 12 may wirelesslycommunicate with programmer 20. For example, IMD 12 may transfer data toprogrammer 20 and may receive data from programmer 20. Programmer 20 mayalso wirelessly program and/or wirelessly charge IMD 12.

Data retrieved from IMD 12 using programmer 20 may include cardiac EGMsstored by IMD 12 that indicate electrical activity of heart 18 andmarker channel data that indicates the occurrence and timing of sensing,diagnosis, and therapy events associated with IMD 12. Data transferredto IMD 12 using programmer 20 may include, for example, operationalprograms for IMD 12 that causes IMD 12 to operate as described herein.

Pacing system 10 may be implanted in right ventricle 16 or anotherchamber of heart 18, using any suitable technique. In some cases, pacingsystem 10 may be implanted according to the method discussed below withrespect to FIGS. 4A-4F and 5. As described in further detail below, insome examples, extension 14 includes a feature configured to facilitatecontrol of the extension during implantation of the extension in theheart. For example, extension 14 may include a housing that defines ahole or extension 14 may otherwise define an opening through which atether may be thread. For example, extension 14 may include an eyeletproximate a distal most electrode of extension 14. A tether may be fedthrough the hole or eyelet during implantation of IMD 12 and extension14 in heart 18. The tether may provide a physical connection toextension 14 that may be used to control the positioning of a distalportion of extension 14. For example, the tether may be used to positiona pusher and a guidewire proximate the distal portion of the guidewire;the pusher and guidewire may then be used to control the position of thedistal most portion of extension 14 relative to heart 18. Afterimplantation, the guidewire, the tether and the pusher are removed frompatient 22.

FIG. 2 is a conceptual illustration of an example pacing system 10 thatincludes IMD 12 and extension 14. IMD 12 is configured to be implantedwithin a chamber of a heart of a patient, e.g., to monitor electricalactivity of the heart and/or provide electrical therapy to the heart. Inthe example shown in FIG. 2, IMD 12 includes outer housing 30, aplurality of fixation tines 32 and electrodes 33 and 34. Extension 14includes electrodes 36 and 38A, 38B, housing 40 and fixation elements42.

Outer housing 30 has a size and form factor that allows IMD 12 to beentirely implanted within a chamber of a heart of a patient. In someexamples, outer housing 16 may have a cylindrical (e.g., pill-shaped)form factor. IMD 12 may include a fixation mechanism configured to fixIMD 12 to cardiac tissue. For example, in the example shown in FIG. 2,IMD 12 includes fixation tines 32 extending from housing 30 andconfigured to engage with cardiac tissue to substantially fix a positionof housing 30 within the chamber of the heart 18. Fixation tines 32 areconfigured to anchor housing 30 to the cardiac tissue such that IMD 12moves along with the cardiac tissue during cardiac contractions.Fixation tines 32 may be fabricated from any suitable material, such asa shape memory material (e.g., Nitinol). Although IMD 12 includes aplurality of fixation tines 32 that are configured to anchor IMD 12 tocardiac tissue in a chamber of a heart, in other examples, IMD 12 may befixed to cardiac tissue using other types of fixation mechanisms, suchas, but not limited to, barbs, coils, and the like.

Housing 30 houses electronic components of IMD 12, e.g., a sensingmodule for sensing cardiac electrical activity via one ore morecombinations of electrodes 33, 34, 36 38A, and 38B and an electricalstimulation module for delivering electrical stimulation therapy via oneor more combinations of electrodes 33, 34, 36, 38A, and 38B. Electroniccomponents may include any discrete and/or integrated electronic circuitcomponents that implement analog and/or digital circuits capable ofproducing the functions attributed to IMD 12 described herein. In someexamples, housing 30 may also house components for sensing otherphysiological parameters, such as acceleration, pressure, sound, and/orimpedance. Although shown with both electrodes 33 and 34, in someexamples, housing 30 may only include one or the other of electrodes 33and 34.

Additionally, housing 30 may also house a memory that includesinstructions that, when executed by one or more processors housed withinhousing 30, cause IMD 12 to perform various functions attributed to IMD12 herein. In some examples, housing 30 may house a communication modulethat enables IMD 12 to communicate with other electronic devices, suchas a medical device programmer. In some examples, housing 30 may housean antenna for wireless communication. Housing 30 may also house a powersource, such as a battery. Housing 30 can be hermetically ornear-hermetically sealed in order to help prevent fluid ingress intohousing 30.

IMD 12 is configured to sense electrical activity of the heart anddeliver electrical stimulation to the heart via one or more combinationsof electrodes 33, 34, 36, 38A, and 38B. In some examples, sensing ofelectrical activity occurs via a first combination of electrodes anddelivery of electrical stimulation occurs via a second, different,combination of electrodes. IMD 12 comprises electrodes 33 and 34 andextension 14 comprises electrodes 36, 38A, and 38B. Although shown withthree electrodes 36, 38A, and 38B in FIG. 2, in some examples, extension14 may only include distal electrode 38A, or may include only electrodes38A, 38B on housing 40.

Electrodes 33 and 34 may be mechanically connected to housing 30. Asanother example, electrodes 33 and 34 may be defined by an outer portionof housing 30 that is electrically conductive. For example, electrode 34may be defined by conductive portion 30A of housing 30. Conductiveportion 30A may, in some examples, define at least part of a powersource case that houses a power source (e.g., a battery) of IMD 12. Thepower source case may house a power source (e.g., a battery) of IMD 12.Electrodes 33 and 34 are electrically isolated from each other.Electrode 33 may be referred to as a tip electrode, and fixation tines32 may be configured to anchor IMD 12 to cardiac tissue such thatelectrode 33 maintains contact with the cardiac tissue. In someexamples, a portion of housing 30 may be covered by, or formed from, aninsulative material to isolate electrodes 33 and 34 from each otherand/or to provide a desired size and shape for one or both of electrodes33 and 34.

Extension 14 is configured to position electrodes 36, 38A, 38B, or justelectrodes 38A, 38B within a chamber other than the one in which IMD 12is implanted. In this way, extension 14 may extend the sensing andpacing capabilities of system 10. In the example shown in FIG. 1,electrode 36 is electrically connected to at least some electronics ofIMD 12 (e.g., a sensing module and a stimulation module) via anelectrical conductor of extension 14 and an electrically conductiveportion 30A of housing 30 of IMD 12, the electrically conductive portion30A being electrically isolated from electrode 33, but electricallyconnected to electrode 34. As a result, electrodes 34, electrode 36 mayhave the same polarity and are electrically common. Conductive portion30A of housing 16 may be electrically connected to at least someelectronics of LPD 12 (e.g., a sensing module, an electrical stimulationmodule, or both), such that conductive portion 30A defines part of anelectrically conductive pathway from electrode 36 to the electronics.

Electrodes 38A, 38B are also electrically connected to at least someelectronics of IMD 12 (e.g., a sensing module and a stimulation module)via one or more electrical conductors of extension 14. In some examples,electrodes 38A, 38B are electrically connected to at least someelectronics of IMD 12 via different electrodes. In addition, in someexamples, one or both of electrodes 38A, 38B may be electrically commonwith electrode 34 and/or electrode 36.

Extension 14 includes a first extension portion 35 and a secondextension portion 37. In the example shown in FIG. 2, electrode 36 iscarried by a first extension portion 35 of extension 14, and is locatedat a distal portion of first extension portion 35, the distal beingfurther from IMD 12 than a proximal portion. In some examples, firstextension portion 35 may be a self-supporting body and is betweenapproximately 5 cm and 12 cm in length. In other examples, however,electrode 36 may have another position relative to first extensionportion 35, such mid-way between housing 30 and the distal end of firstextension portion 35, or otherwise away from the distal end of firstextension portion 35. In some examples, a center of electrode 36 may bebetween approximately 3 cm and 10 cm from the distal end of IMD 12(i.e., the end closest to extension 14). In some examples, a center ofelectrode 36 may be approximately 10 cm from the distal end of IMD 12.

In some examples, IMD 12 senses electrical activity of right atrium 24of heart 18 of patient 22 via electrode 36; extension 14 may beconfigured such that when IMD 12 is implanted in right ventricle 16,electrode 36 is located within right atrium 24. Electrode 36 may haveany suitable configuration. For example, electrode 36 may have aring-shaped configuration, or a partial-ring configuration. Electrode 36may be formed from any suitable material, such as a titanium nitridecoated metal.

Second extension portion 37 is distal to first extension portion 35,such that first extension portion 35 is positioned between IMD 12 andsecond extension portion 37. In some examples, second portion 37 has across-sectional dimension (e.g., a diameter) approximately equal to thediameter of first portion 35. For example, the diameter of secondextension portion 37 may be approximately 4 French. In other examples,second extension portion 37 has a cross-sectional dimension smaller thenthe cross-sectional dimension of first extension portion 35. Forexample, the diameter of second extension portion 37 may beapproximately 2 French.

In some examples, first and second extension portions 35, 37 areconfigured such that when IMD 12 is implanted in right ventricle 16,electrodes 38A, 38B and housing 40 of extension 14 are implanted in aleft ventricle of heart 18. For example, the length of second extensionportion 37 may be approximately 15 cm to 20 cm, and extension 14 mayhave a total length of approximately 20 cm to 25 cm.

Electrodes 38A, 38B are located at the distal end of second extensionportion 37 on a housing 40. Housing 40 may include a hole or eyelet thatincludes a central axis H_(A) that is parallel to a longitudinal axisE_(A) of extension 14. The hole or eyelet may be configured to receivetether and/or a guidewire. Electrodes 38A, 38B may have any suitableconfiguration. For example, electrodes 38A, 38B may each be aring-shaped configuration, or a partial-ring configuration. Electrodes38A, 38B may be formed from any suitable material, such as a titaniumnitride coated metal. Although housing 40 including two electrodes 38A,38B are shown in FIG. 2, in other examples of system 10, housing 40 mayinclude any suitable number of electrodes, such as one or more than twoelectrodes.

Second portion 37 of extension 14 may also include one or more fixationelements 42 configured to engage with tissue to substantially fix aposition of electrodes 38A, 38B relative to heart 18 of patient 22.Fixation elements 42 may be any suitable type of fixation element, suchas tines, double-bended elements, s-curve element, barbs, coils, or thelike.

The relative spacing between electrodes 36, 38A, 38B may vary based onthe type of stimulation and/or sensing system 10 is configured toprovide. In some examples, electrode 36 is spaced from electrodes 38A,38B along extension 14 such that when implanted system 10 is implantedin heart 22, electrode 36 is located within the right atrium, andelectrodes 38A, 38B are located within the coronary sinus proximate tothe left ventricle. In this example, system 10 may be used to senseelectrical activity in both right ventricle 16 and the left ventricle,as well as deliver electrical stimulation to both right ventricle 16 andleft ventricle, e.g., to provide biventricular pacing therapy.

FIG. 3 is a functional block diagram of an example IMD 12. IMD 12includes a processing module 50, memory 52, stimulation module 54,electrical sensing module 56, communication module 58, sensor 60, andpower source 62. Power source 62 may include a battery, e.g., arechargeable or non-rechargeable battery.

Modules included in IMD 12 represent functionality that may be includedin IMD 12 of the present disclosure. Modules of the present disclosuremay include any discrete and/or integrated electronic circuit componentsthat implement analog and/or digital circuits capable of producing thefunctions attributed to the modules herein. For example, the modules mayinclude analog circuits, e.g., amplification circuits, filteringcircuits, and/or other signal conditioning circuits. The modules mayalso include digital circuits, e.g., combinational or sequential logiccircuits, memory devices, and the like. The functions attributed to themodules herein may be embodied as one or more processors, hardware,firmware, software, or any combination thereof. Depiction of differentfeatures as modules is intended to highlight different functionalaspects, and does not necessarily imply that such modules must berealized by separate hardware or software components. Rather,functionality associated with one or more modules may be performed byseparate hardware or software components, or integrated within common orseparate hardware or software components.

Processing module 50 may include any one or more of a microprocessor, acontroller, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), orequivalent discrete or integrated logic circuitry. In some examples,processor 50 may include multiple components, such as any combination ofone or more microprocessors, one or more controllers, one or more DSPs,one or more ASICs, or one or more FPGAs, as well as other discrete orintegrated logic circuitry.

Processing module 50 may communicate with memory 52. Memory 52 mayinclude computer-readable instructions that, when executed by processingmodule 50, cause processing module 50 to perform the various functionsattributed to processing module 50 herein. Memory 52 may include anyvolatile, non-volatile, magnetic, or electrical media, such as a randomaccess memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM),electrically-erasable programmable ROM (EEPROM), Flash memory, or anyother memory device. Furthermore, memory 52 may include instructionsthat, when executed by one or more processors, cause the modules toperform various functions attributed to the modules herein. For example,memory 52 may include pacing instructions and values. The pacinginstructions and values may be updated by programmer 20 (FIG. 1).

Stimulation module 54 and electrical sensing module 56 are electricallycoupled to electrodes 33, 34, 36, 38A, 38B. Processing module 50 isconfigured to control stimulation module 54 to generate and deliverelectrical stimulation to heart 18 (e.g., right ventricle 16 and theleft ventricle via coronary sinus 26 in the example shown in FIG. 1) viaa selected subset of electrodes 33, 34, 36, 38A, 38B. For example,stimulation module 54 may deliver pacing electrical stimulation to rightventricle 16 via electrodes 33 and 34, which are configured to beimplanted within right ventricle 16. As another example, stimulationmodule 54 may deliver pacing electrical stimulation to the leftventricle via electrodes 38A, 38B, which are configured to be implantedwithin the left ventricle. Electrical stimulation may include, forexample, pacing pulses, or any other suitable electrical stimulation.Processing module 50 may control stimulation module 54 to deliverelectrical stimulation therapy via a selected subset of electrodes 33,34, 36, 38A, 38B according to one or more therapy programs includingpacing instructions that define a ventricular pacing rate, which may bestored in memory 52.

In addition, processing module 50 is configured to control electricalsensing module 56 monitor signals from any suitable subset of electrodes33, 34, 36, 38A, 38B in order to monitor electrical activity of heart18. For example, electrical sensing module 56 may sense electricalactivity within right ventricle 16 via electrodes 33, 34, may senseelectrical activity within right atrium 24 via electrodes 33, 36, maysense electrical activity within the right ventricle via electrodes 38A,38B, or any combination of the aforementioned sensing may be performedby sensing module 56.

Electrical sensing module 56 may include circuits that acquireelectrical signals. Electrical sensing module 56 may acquire electricalsignals form a subset of electrodes 33, 34, 36 and 38A, 38B. Electricalsignals acquired by electrical sensing module 56 may include intrinsiccardiac electrical activity, such as intrinsic atrial depolarizationand/or intrinsic ventricular depolarization. Electrical sensing module56 may filter, amplify, and digitize the acquired electrical signals togenerate raw digital data. Processing module 50 may receive thedigitized data generated by electrical sensing module 56. In someexamples, processing module 50 may perform various digital signalprocessing operations on the raw data, such as digital filtering.

Processing module 50 may sense cardiac events based on the data receivedfrom electrical sensing module 56. For example, processing module 50 maysense atrial electrical activity based on the data received fromelectrical sensing module 56. For example, in examples in which IMD 12and extension 14 are implanted in right ventricle 16, processing module50 may detect P-waves indicative of atrial activation events based onthe data received from electrical sensing module 56 via electrodes 33,36. In some examples, processing module 50 may also sense ventricularelectrical activity based on the data received from electrical sensingmodule 56. For example, processing module 50 may detect R-wavesindicative of right ventricular activation events based on the datareceived from electrical sensing module 56 via electrodes 33, 34. Inexamples in which processor 50 uses multiple electrodes for both R-waveand P-wave sensing, processor 50 may detect the R-waves and P-waves fromthe same sensed signal. The sensing vector can be between electrodes 33and 36, for example. As another example, processing module 50 may detecta signal characteristic indicative of left ventricular activation eventsbased on the data received from electrical sensing module 56 viaelectrodes 38A, 38B, or via electrodes 36, 38A, or via electrodes 36,38B.

In some examples, in addition to electrical sensing module 56, IMD 12includes sensor 60, which may comprise at least one of a variety ofdifferent sensors. For example, sensor 60 may comprise at least one of apressure sensor and an accelerometer. Sensor 60 may generate signalsthat indicate at least one of parameter of patient 22, such as, but notlimited to, at least one of: an activity level of patient 22, ahemodynamic pressure, and heart sounds.

Communication module 58 may include any suitable hardware (e.g., anantenna), firmware, software, or any combination thereof forcommunicating with another device, such as programmer 20 (FIG. 1) or apatient monitor. Under the control of processing module 50,communication module 58 may receive downlink telemetry from and senduplink telemetry to other devices, such as programmer 2 or a patientmonitor, with the aid of an antenna included in communication module 58.

FIGS. 4A-4F illustrate a method of implanting system 10, and, inparticular, housing 40 of extension 14 in a desired location withinheart 18, e.g., to position one or both electrodes 38A, 38B to senseelectrical activity of a particular chamber (e.g., the left ventricle)or deliver electrical stimulation to the particular chamber. In FIG. 4A,IMD 12 has been implanted in the right ventricle 16 (FIG. 1), andextension 14 enters right atrium 24 (FIG. 1) via tricuspid valve 64. Atether 69 comprising a first tether portion 66 and a second tetherportion 68 is threaded through a hole in housing 40, the hole having acentral axis that is substantially parallel (e.g., parallel or nearlyparallel) to the longitudinal axis of extension 14. In some examples,first tether portion 66 is positioned on one side of housing 40 andextends from one end of the hole, and the second tether portion 68 is onthe other side of housing 40 and extends form the other end of the hole.

In some examples, the first tether portion 66 is a different color thansecond tether portion 68 in order to help the clinician visuallydistinguish between the tether portions 66, 68. Tether 69 may be placedwithin the hole in housing 40 prior to implantation. For example, amanufacturer of extension 14 may install tether 69 in the hole ofhousing 40.

In some examples, housing 40 and tether 69 may be introduced into heart18 via introducer 70. Introducer 70 may be introduced to heart 18 viathe femoral vein (not shown). In the example shown in FIG. 4A, housing40 and tether 69 are located within introducer 70. Introducer 70 issized to receive extension 14 and tether 69, but is still relativelysmall in order to reduce the invasiveness of introducer 70. In someexamples, introducer 70 may have a diameter of approximately 22 French,although other dimensions are contemplated. The diameter of introducer70 may be the diameter of a cross-section of introducer 70 taken alongan axis perpendicular to a longitudinal axis of introducer 70.

Guidewire 72 and catheter 74 may be used to define a pathway forextension 14 from introducer 70 to a location within coronary sinus 26or another location within heart 18. In the example shown in FIG. 4B,guidewire 72 is introduced into a target vein 76 using a catheter 74,which may be guided to a location proximate coronary sinus 26 viaintroducer 70. In the example shown, target vein 76 branches from thecoronary sinus 26. In some examples, the target vein is the coronaryvein of the left ventricle.

Guidewire 72 and catheter 74 may have any suitable configuration. Insome examples, guidewire 72 has a proximal guidewire portion and adistal guidewire portion having a different configuration than theproximal guidewire portion. The distal guidewire portion may be theportion of guidewire 72 that is furthest from the clinician whenguidewire 72 is being inserted through tissue. In some examples, thedistal guidewire portion (e.g., the distal most 20 cm) of guidewire 72may be configured as a standard guidewire formed from any suitable wire(e.g., a nickel titanium wire). As an example, the distal guidewireportion may be formed from a wire that tapers in a distal direction froma diameter of about 0.014 inches to a diameter of about 0.012 inches,and becomes more flexible as it tapers. Other guidewire configurationsand other guidewire dimensions may also be used, and may vary based onthe implant location of housing 40 of extension 14.

In some examples, the proximal guidewire portion of guidewire 72 may bea suture thread, e.g., a 0.010 cm diameter suture thread, rather than awire. To enable guidewire 72 to be pushed, rotated and pulled, theproximal guidewire portion may be is temporarily stiffened with astiffening tube within which the proximal guide portion may beintroduced. Catheter 74 may be configured to receive the stiffeningtube. In some examples, the stiffening tube is formed from abiocompatible plastic material and has a 0.012 cm inner diameter, and a0.030 outer diameter.

After the distal end of guidewire 72 is at target vein 76, as shown inFIG. 4C, both the stiffening tube (if used) and catheter 74 may beremoved from patient 22, leaving guidewire 72 in place. In the stepshown in FIG. 4D, first tether portion 66 is attached to the proximalend of the guidewire 72 (e.g., attached to the proximal guidewire'sthread-like portion) at attachment point 80. For example, the proximal,threadlike, portion of guidewire 72 (portion 75 shown in FIG. 8, below)is intertwined with first tether portion 66 at point 80 to form onecontinuous thread. In some examples, a drop of instant adhesive may beapplied in order to insure the integrity of the attachment.

A clinician may introduce pusher 78 into introducer 70. In someexamples, pusher 78 is made of wire. Pusher 78 may include eyelet 79 oranother feature at a distal end 78A that is sized to receive tether 68,and engage with housing 40 to apply a force against housing 40 to movehousing 40 in a direction away from introducer 70. In some examples,pusher 78 is made of stainless steel wire. In some examples, pusher 78may be approximately 0.020 inches in diameter. In some examples, eyelet79 is defined by one or more turns of the wire from which pusher 78 isformed. As shown in FIG. 4D, second tether portion 68 is threadedthrough eyelet 79 of pusher 78, thereby securing the relative lateralposition between pusher 78 and tether 69. Pusher 78 may be moved alongtether 69 (i.e., in a direction along a longitudinal axis of tether 69)to reach housing 40.

A clinician may pull on second tether portion 68, in a direction awayfrom housing 40 (and, in some examples, in a direction away from patient22). In some examples, the clinician may hold housing 40 in place withpusher 78 as second tether portion 68 is pulled. Prior to pulling ontether 69, tether 69 was extending through the hole defined by housing40. As the clinician pulls on second tether portion 68, first tetherportion 66 is pulled through the hole define by housing 40, such thatthe proximal guidewire portion of guidewire 72, which is attached tofirst tether portion 66, is fed through the hole defined by housing 69.In this way, guidewire 72 may replace tether 69, as shown in FIG. 4E.The proximal guidewire portion may be pulled through the hole defined byhousing 69 because it is formed from a relatively flexible material,e.g., a suture thread, and not from the stiffer wire material with whichfirst guidewire portion is formed from.

Guidewire 72 may be used to guide housing 40 to target vein 76. Asdescribed with respect to FIGS. 4A-4D, tether 69 provides a relativelyquick and simple way to thread guidewire 72 through the hole in housing40 while housing 40 is implanted in patient 22.

As further shown in FIG. 4E, pusher 78 is used to push housing 40 alongguidewire 72 towards target vein 76 (or other desired location). In someexamples, target vein 76 may be selected in order to providebiventricular pacing. As the electrodes 38A, 38B and housing 40approaches the target vein 76, fine adjustment of the location ofelectrodes 38A, 38B relative to specific locations of heart 18 may beachieved by pushing on pusher 78 to move electrodes 38A, 38B distally(away from introducer 70), and by pulling on guidewire 72 and pusher 89to move electrodes 38A, 38B in a proximal direction, towards introducer70. After housing 40 is positioned proximate target vein 76 such thatelectrodes 38A, 38B are at a suitable location (e.g., as confirmed byany suitable testing of stimulation delivered via electrodes 38A, 38B orsensed by electrodes 38A, 38B) for delivering the pacing therapy to theleft ventricle, pusher 78, guidewire 72, and introducer 70 are removedfrom patient 22. FIG. 4F depicts an example implantation setup of system10.

FIG. 5 is a flowchart illustrating an example method for implantingcardiac pacing system 10 in a heart 18 of patient 22 to providebiventricular pacing. A clinician introduces an IMD 12 to the rightventricle 16 of patient 22 (82). In some examples IMD 12 may beintroduced into the heart 18 via a femoral vein. In some examples, priorto implantation, pacing system 10 includes a tether, such as tether 69,which is threaded through a hole in housing 40 of extension 14. Theclinician directs a guidewire 72 to a target vein (84). In someexamples, the target vein may be off of the coronary sinus, and locatedso that when in place, electrodes 38A, 38B on housing 40 of extension 14may provide pacing stimulation to the left ventricle. In some examples,a proximal end of the guidewire 72 may be attached to the first tetherportion 66 of tether 69.

In some examples, the clinician introduces a pusher 78 along the secondtether portion 68 of the tether to housing 40 (86). The clinician maythe remove tether 69 from housing 40 by pulling the second tetherportion 68 (88). Pulling on the second portion 68 of the tether resultsin guidewire 72 replacing tether 69 within the hole in housing 40. Theclinician may then push housing 40 towards the target vein along theguidewire 72 using the pusher 78 (90). Pusher 78 and guidewire 72 areused to position electrodes 38A, 38B on housing 40 at a desiredlocation, which may be selected in order provide adequate pacingstimulation to the left ventricle of heart 18.

FIG. 6 is a conceptual illustration of a distal housing 40. Distalhousing 40 includes electrodes 38A and 38B and hole 41. Hole 41 isconfigured to accept tether 69 and/or guidewire 72. The longitudinalaxis of hole 41 is parallel to the longitudinal axis E_(A) of extension14.

FIG. 7 is a conceptual illustration of a cutaway of a distal housing 40at the distal end of second extension portion 37. The cutaway of housing40 shows hole 41. The longitudinal axis of hole 41 is parallel to thelongitudinal axis of the second extension portion 37.

FIG. 8 is a conceptual illustration of guidewire 72 and stiffening tube77. Guidewire 72 includes first guidewire portion 73 and a secondguidewire portion 75. The first guidewire portion 73 resembles astandard guidewire and can be formed from a standard guidewire in someexamples. In some examples, the first guidewire portion 73 isapproximately 20 cm in length. The first guidewire portion may taperfrom 0.014 inches to 0.012 inches and becomes more flexible as ittapers. In some examples, the second guidewire portion 75 may be athread. The thread may be approximately 0.010 inches in diameter. Thesecond guidewire portion 75 may be encompassed by stiffening tube 77.Stiffening tube may be made of plastic. In some examples, stiffeningtube 77 may have a 0.012 inch inside diameter, and a 0.030 inch outsidediameter.

The techniques described in this disclosure, including those attributedto IMD 12, programmer 20, or various constituent components, may beimplemented, at least in part, in hardware, software, firmware or anycombination thereof. For example, various aspects of the techniques maybe implemented within one or more processors, including one or moremicroprocessors, DSPs, ASICs, FPGAs, or any other equivalent integratedor discrete logic circuitry, as well as any combinations of suchcomponents, embodied in programmers, such as physician or patientprogrammers, stimulators, image processing devices or other devices. Theterm “processor” or “processing circuitry” may generally refer to any ofthe foregoing logic circuitry, alone or in combination with other logiccircuitry, or any other equivalent circuitry.

Such hardware, software, firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as RAM, ROM, NVRAM,EEPROM, FLASH memory, magnetic data storage media, optical data storagemedia, or the like. The instructions may be executed to support one ormore aspects of the functionality described in this disclosure.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A system comprising: an implantable medicaldevice configured to be implanted in a chamber of a heart of a patient;an extension attached to the implantable medical device, the extensioncomprising a housing comprising at least one electrode, the housingdefining a hole; and a tether comprising a first tether portion and asecond tether portion and configured to be threaded through the hole,wherein when the tether is threaded through the hole, the first tetherportion and the second tether portion are on opposite sides of the hole.2. The system of claim 1, further comprising a guidewire comprising afirst guidewire portion and a second guidewire portion, the secondguidewire portion comprising a thread attached to the second tetherportion.
 3. The system of claim 2, further comprising a stiffening tubeconfigured to receive the guidewire.
 4. The system of claim 2, furthercomprising a catheter configured to receive guidewire.
 5. The system ofclaim 1, further comprising a pusher comprising a wire with an eyelet ata distal end of the wire, the eyelet configured to encompass the tetherand push the housing.
 6. The system of claim 1, wherein the first tetherportion is a first color, and the second tether portion is a secondcolor.
 7. The system of claim 1, wherein the housing of the extensioncomprises a fixation element configured to engage with tissue of thepatient.
 8. The system of claim 1, wherein housing is a first housingand the at least one electrode comprises a first electrode, theextension further comprising a second housing comprising a secondelectrode.
 9. The system of claim 8, wherein the implantable medicaldevice comprises a third electrode, and wherein the first electrode, thesecond electrode and the third electrode are spaced relative to eachother so that when the implantable medical device and the extension areimplanted within the heart, the first electrode is located in a leftcoronary vein, and the third electrode is located in the rightventricle.
 10. The system of claim 1, wherein the extension defines alongitudinal axis, and the hole extends along an axis of the housingthat is substantially parallel to the longitudinal axis of theextension.
 11. A method comprising: introducing an implantable medicaldevice to a first chamber of a heart of a patient via a femoral vein,wherein the implantable medical device is attached to an extensioncomprising a housing including at least one electrode, the housingdefining a hole, and wherein a tether is threaded through the hole, thetether including a first tether portion and a second tether portion onan opposite side of the hole from the first tether portion; directing aguidewire to a target vein, the guidewire comprising a first guidewireportion and a second guidewire portion, the second guidewire portioncomprising a thread attached to the second tether portion; introducing apusher up the second tether portion to a position proximate to thehousing; removing the tether from the patient by at least pulling thesecond tether portion to replace the tether with the guidewire; pushingthe housing along the guidewire using the pusher.
 12. The method ofclaim 11, wherein pushing the housing along the guidewire using thepusher comprises pushing the housing with the pusher to a location in acoronary sinus of the patient.
 13. The method of claim 11, furthercomprising adjusting a location of the at least one electrode within thepatient by at least one of: pushing the pusher to move the housing in adistal direction, or pulling on both the guidewire and the pusher tomove the housing in the proximal direction.
 14. The method of claim 13,wherein the at least one electrode comprises a first electrode and thefirst chamber comprises a right ventricle, the implantable medicaldevice comprising a second electrode, wherein adjusting the location ofthe at least one electrode comprises adjusting the location of the firstelectrode to be positioned in a left ventricle of the heart of thepatient.
 15. The method of claim 11, wherein the thread is encompassedby a stiffening tube, the method further comprising removing thestiffening tube from around the thread.
 16. The method of claim 11,wherein introducing the implantable medical device into the firstchamber of the heart of the patient comprises: introducing a catheterinto the chamber of the heart of the patient via the femoral artery; andintroducing the implantable medical device into the chamber through thecatheter.